1. Field of the Invention
This invention relates to a control apparatus for a variable-speed hydraulic power generating system, which controls the opening of a guide vane for controlling the operation of a water turbine so as to rotate the water turbine at an optimum rotation speed.
2. Description of the Related Art
One of principal objects of controlling the rotation speed of a rotary electric machine is to control the rotation speed of a turbo-machine such as a reversible pump-turbine according to the load of the turbo-machine thereby achieving the operation of the turbo-machine at the highest efficiency. A water turbine in a hydraulic power generating system is operated at a variable speed by one of two broadly classified methods.
According to the first method, a frequency converter is provided between an a.c. system and an electric generator. JP-A-48-21045 proposes a method according to which electric power can be supplied to an a.c. system regardless of operation of an electric generator at any rotation speed, and a guide vane of a water turbine is suitably opened and closed to regulate the rotation speed of the water turbine thereby achieving the operation of the water turbine at the point of its highest efficiency.
According to the second method, a wound-rotor induction machine is connected at its primary side to an a.c. system, and a frequency converter is provided between the a.c. system and the secondary side of the induction machine. This method was already known as a typical application of a method in which a wound-rotor induction machine is connected at its primary side to an a.c. system and has a frequency converter connected between its secondary side and the a.c. system so as to control the rotation speed of a water turbine according to the generator output. Such a method is described in, for example, "Handbook of Electrical Engineers" Part 14, Section 5.4.2 (d) in page 734 published by the Institute of Electrical Engineers of Japan in 1967. A control apparatus for such a variable-speed hydraulic power generating system is proposed in, for example, JP-A-52-46428, JP-A-55-56499 or JP-A-59-72998.
The problem common to the two kinds of control apparatus for variable-speed hydraulic power generating systems as described above is how the water turbine output and the electric generator output are to be controlled for controlling the rotation speed of the water turbine. More concretely, an optimum rotation speed Na is calculated on the basis of signals commanding operating conditions of the water turbine, which signals include at least an externally applied power generation command signal Po, and the calculated optimum rotation speed Na is compared with an actually detected value N of the rotation speed of the water turbine to obtain a speed error signal (Na-N). The problem is how this speed error signal (Na-N) is used for controlling both the water turbine output and the electric power generator. The power generation command signal Po is externally applied from a central load-dispatching office which determines the power output to be generated by the hydraulic power generating system on the basis of the power demanded by an a.c. electric power system to which the hydraulic power generating system is connected.
In connection with a control apparatus based on the second method described above in which a wound-rotor induction machine is connected at its primary side to an a.c. system and has a frequency converter connected between its secondary side and the a.c. system for controlling the operation of a water turbine in a variable-speed hydraulic power generating system, it is disclosed in U.S. Pat. No. 4,694,189 issued Sept. 15, 1987, granted on U.S. patent application Ser. No. 911,131 to control, on one hand, the power generation by a generator excitation control unit on the basis of a power generation command signal Po and to control, on the other hand, the opening of the guide vane of the water turbine on the basis of a speed error signal (Na-N).
In the control apparatus of the kind described above, the power generation increases while substantially following up the power generation command signal Po which is a stepwise increasing signal. However, the response of the guide vane opening control to the speed error signal (Na-N) is slow relative to the response of the power generation control to the power generation command signal Po. Therefore, the increase in the water turbine output is delayed relative to the increase in the power generation, with the result that the rotation speed of the water turbine is temporarily decreased to increase the value of the speed error signal (Na-N), thereby tending to further increase the opening of the guide vane. Thus, the response of the water turbine output under control of the mechanical system is slow relative to the response of the power generation under control of the electrical system. Therefore, in some cases, undesirable resonance may possibly occur in the mechanical system. Occurrence of such resonance in the mechanical system may result in build-up of an unusually high water pressure in the penstock to the water turbine thereby giving rise to a great variation of the rotation speed of the water turbine. Also, since the resonance frequency is low, the power generation may vary in the vicinity of the low resonance frequency to such an extent that it cannot follow up the generation output command.